The present application applies to a method and apparatus for acquiring data that may be synthesized through tomosynthesis. Tomosynthesis ordinarily provides means for two-dimensional (2-D) images of an object, taken at a plurality of angles, to be combined and synthesized into a plurality of 2-D images that represent various tomography planes (e.g., slices) of the object. While the techniques disclosed herein may be applied in a variety of fields, they find particular application in the medical field.
Radiation devices, in general, provide a means for generating 2-D images of an object under examination. The object is exposed to radiation, and a 2-D image is formed based upon the radiation absorbed by the object, or rather a level of radiation (e.g., energy) that is able to pass through the object. Highly dense objects absorb more radiation than less dense objects, and thus an object having a higher density, such as a mass (e.g., a benign cyst or tumor), for example, will be apparent when surrounded by less dense objects, such as fatty tissue or muscle. In medical systems, radiation devices are commonly used to detect broken bones, masses, calcium deposits, etc. that are ordinarily not visible.
A radiation device typically comprises a detector array and a radiation source mounted on a diametrically opposing side of the object from the detector array, where the radiation source emits radiation toward the object. In digital radiology, digital pixels (of the detector array) detect radiation that traverses the object, and reconstruction algorithms create 2-D images of the object in the latitudinal dimension (e.g., orthogonal to a center ray and parallel to the detector array) based upon the energy of photons comprised in the detected radiation.
While 2-D images are useful in some applications, such as to detect a broken bone, the 2-D images are less useful for other procedures, such as breast cancer detection, because the images have no resolution in the longitudinal dimension (e.g., parallel to the center ray and orthogonal to the detector array). On a breast examination, for example, the 2-D images cannot provide information about whether a mass has ramified (e.g., spread to ducts in the breast and is likely to be malignant) in a longitudinal direction. Additionally, a less dense, but potentially cancerous mass, for example, may be masked by a more dense target, such as scar tissue, if the mass and scar tissue have similar latitudinal coordinates (e.g., one target is on top of the other).
Digital tomosynthesis, as described in U.S. Pat. No. 6,960,020 to Lai, enables a substantially three-dimensional (3-D) view of an object to be constructed from a finite set of 2-D images of the object. Typically, in digital tomosynthesis systems, the position of the radiation source is varied during data acquisition (with respect to the detector array and/or the object), for example in an arc through a limited angular range, and a set of projections of the object are acquired. A projection may represent data related to radiation emitted while the radiation source was at a predetermined point along the arc (e.g., a tomosynthesis view of the object). The set of projections may be converted into a set of 2-D images (through reconstruction) and combined and/or filtered, using digital tomosynthesis algorithms, to produce a certain degree of resolution in the longitudinal dimension of the object. That is, a set of synthesized 2-D images may be produced representing a plurality of tomography planes (e.g., slices) of the object. It will be appreciated that the number of synthesized 2-D images may be a function of the number of tomosynthesis views of the object. For example, a larger number of tomosynthesis views may promote a higher resolution in the longitudinal direction so the synthesized images may depict thinner tomography planes without the image becoming distorted because of smearing.
While current tomosynthesis acquisition techniques (e.g., based upon 2-D images acquired by measuring photon energy of the detected radiation) have proven effective in some instances, there remains room for improvement. The number of 2-D images (depicting a variety of tomosynthesis views) acquired by the apparatus is limited because the measure of radiation energy adds an amount of electronic noise (e.g., more tomosynthesis views add more electronic noise). While it is desirable to portion the examination dose over a large number of tomosynthesis views (thus allowing a higher longitudinal resolution), because electronic noise is additive (for each additional tomosynthesis view) the number of views is limited to provide a signal-to-noise ratio above some threshold. Additionally, the angular separation between tomosynthesis views (e.g., the number of degrees the radiation source is rotated) is restricted to promote image quality and/or reduce reconstruction artifacts. Therefore, the number of tomosynthesis views that allow a signal-to-noise ratio above some threshold (e.g., twenty views) and a permissible angular separation there-between (e.g., two degrees) define an allowable tomosynthesis angle (e.g., ±twenty degrees from center). This may, however, be less than ideal where, for example, a greater angle is desired (e.g., ±forty degrees from center).
Current tomosynthesis procedures also rely upon the radiation source travelling along a predetermined trajectory at a relatively slow, continuous speed while radiation is pulsed (e.g., emitted at predetermined points along the trajectory) to generate the images. While a faster speed may decrease the probability of reconstruction artifacts (due to a patient's movement), a faster speed (without reducing pulse duration) may also cause a virtual “elongation” of the focal spot of the radiation source and/or a tangential defocusing (e.g., poor focus) of the synthesized image, thereby reducing image quality. While the virtual “elongation” may be partially cured by increasing power to the radiation source, increasing the power also causes the size of actual focal spot to increase and the quality of the images to decrease. Therefore, the size of actual focal spot and the virtual “elongation” of the focal spot are balanced to improve image quality.